Amino acids

The administration of individual sulfur-containing amino acids has been shown to abrogate the impaired healing in protein-deficient rats, as evidenced by increased fibroblastic proliferation and collagen accumulation.11,12

Recently, acetylcysteine is being investigated with respect to its ability to squelch oxygen radicals and provide antioxidant action for wound healing.13

The branched-chain amino acids (valine, leucine, and isoleucine) have been used to treat liver disease and have an additional role in retaining nitrogen in sepsis, trauma, and burns. Branched-chain amino acids support protein synthesis after injury and decrease muscle proteolysis. Serving as caloric substrates, branched-chain amino acids can be metabolized as an energy source independent of liver function.14-21

Glutamine is the most abundant amino acid in the body, and it accounts for approximately 20% of the total circulating free amino acid pool and 60% of the free intracellular amino acid pool. The process of gluconeogenesis involves the shuttling of alanine and glutamine to the liver for conversion to glucose, which is used peripherally as fuel to power certain aspects of wound healing. Glutamine is also an important precursor for the synthesis of nucleotides in cells, including fibroblasts and macrophages. Glutamine is an energy source for lymphocytes and is essential for lymphocyte proliferation. Finally, glutamine has a crucial role in stimulating the inflammatory immune response occurring early in wound healing.

Given the abundant roles of glutamine in the numerous cells involved in wound healing, it is not surprising that after injury there is a rapid fall in plasma and muscle glutamine levels, which is greater than that of any other amino acid. Although efficacy of supplemental glutamine administration has been shown in some clinical situations, it has not been proved to have any dramatic effect on wound healing.24-34

Arginine is absorbed from the intestine by a transport system shared with lysine, ornithine, and cysteine in an energy-dependent and sodium-dependent fashion with substrate specificity. Arginine also shares a common uptake and transport system into fibroblasts and leukocytes with these amino acids.35

Arginine is synthesized in adequate quantities to sustain muscle and connective tissue mass but in insufficient quantities for optimal protein biosynthesis and healing. In situations of stress or injury, in which synthesis of arginine is insufficient to meet the demands of increased protein turnover, arginine becomes an indispensable amino acid in the process of wound healing and maintenance of a positive nitrogen balance.

Arginine supplementation has no effect on the rate of epithelialization of the skin defect; the predominant effect of arginine is on wound collagen deposition. The beneficial effects of supplemental arginine on wound healing are similar to the effects of growth hormone; specifically, enhanced wound breaking strength and collagen deposition.36-38 Arginine has been identified as the unique substrate for the generation of the highly reactive radical nitric oxide (NO). Several studies suggest that NO plays a crucial role in wound healing. Inhibitors of NO have been shown to significantly impair healing of cutaneous incisional wounds and colonic anastomosis in rodents.39 40 The inducible NO synthase (iNOS) pathway is at least partially responsible for the enhancement of wound healing observed with the administration of arginine.41 Nitric oxide is synthesized by the enzyme nitric oxide synthase, which converts the amino acid L-arginine to citrulline and NO. NO functions in biological systems in two very important ways. First, it has been found to be a messenger by which cells communicate with one another (signal transduction); second, it plays a critical role in the host response to infection. In this second function, it appears that the toxic properties of NO have been harnessed by the immune system to kill or at least slow the growth of invading organisms. The nonspecific chemical reactivity with key cellular targets is responsible for this action. In signaling, NO directly activates the enzyme soluble guanylate cyclase (sGC). Once activated, sGC converts cyclic guanosine triphosphate (GTP) to cyclic guanosine monophosphate (cGMP) and pyrophosphate. The cGMP formed is responsible for the well-documented actions of NO, such as blood vessel dilation. With the initial discovery of NO signaling, several important questions emerged that centered largely on the issue of how a signaling system functions when the signaling agent is chemically reactive (short lived), highly diffusible, and toxic. Critical, especially in signaling, are the control of NO biosynthesis and interaction with the biological receptors in due time at a concentration that will not harm the host.42

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